The present invention relates to a circuit incorporated IGBT in which an IGBT and a circuit are formed in a semiconductor substrate.
An insulated gate bipolar transistor (hereinafter abbreviated to IGBT) is a voltage controlled switching element which can control a current of a main terminal by a voltage of a control terminal. Since a large current and switching at a high frequency are possible, IGBT""s are being used at the present time in a wide field of applications from household air conditioners to inverters of electric cars and so forth.
Hitherto, the contemplation of a low loss and a high speed was made for IGBT""s. In recent years, not only the low loss and the high speed but also a high functionality have been promoted. A high-function IGBT includes, for example, an IGBT in which the IGBT is integrated with a protection circuit so that a protecting function is possessed by one chip. A problem encountered in integrating an IBGT with a circuit is an erroneous operation of the circuit caused by a carrier current or hole current peculiar to the IGBT. When a hole current injected from a collector layer of the IGBT flows into a circuit area, an erroneous operation of the circuit occurs. There has been disclosed a structure in which a layer for ejecting holes is provided in order to prevent the hole current from flowing into the circuit area.
FIG. 11 shows the cross-sectional structure of a circuit integrated IGBT having a hole ejecting layer. In FIG. 11, reference numeral 101 denotes a collector layer, numeral 102 a buffer layer, numeral 103 a drift layer, numeral 104 a channel layer, numeral 105 an emitter layer, numeral 106 a hole ejecting layer, numeral 110 an emitter electrode, numeral 111 a gate electrode, numeral 112 a gate oxide film, numeral 114 a source electrode, numeral 115 a MOSFET gate electrode, numeral 116 a drain electrode, numeral 117 a collector electrode, numeral 131 a source layer, numeral 132 a base layer, numeral 133 a drain layer, numeral 150 an IGBT area, numeral 151 a circuit area, and numeral 152 a lateral MOSFET. Though not shown in FIG. 11, resistors, diodes and so forth are formed as circuit forming elements other than the MOSFET in the circuit area 151. Similarly, though not shown, the source electrode 114, gate electrode 115 and drain electrode 116 of the MOSFET are connected to the other elements formed in the circuit area and the emitter electrode 110 and gate electrode 111 of the IGBT. In a turned-on condition of the IGBT, a hole current flows from the collector layer to the emitter layer, as shown by arrow in FIG. 11. In order to suppress the flow of this hole current into the circuit area, the hole ejecting layer 106 is provided to prevent the hole current from flowing from the IGBT area into the circuit area.
In recent years, however, the high functionality and high preciseness of a circuit integrated in an IGBT have been advanced and there has been generated a problem that an erroneous operation of the circuit occurs even with a very small leakage current of holes. This is because even if the hole ejecting layer 106 is provided, a very small amount of holes may leak into the circuit area. Such an erroneous operation is remarkable in the case where an IGBT is integrated with a source follower circuit using a MOSFET.
FIGS. 12 and 13 show the cross section and the equivalent circuit of an IGBT integrated with a source follower circuit. In FIGS. 12 and 13, the same constituent elements as those in FIG. 11 are denoted by the same reference numerals as those used in FIG. 11. In FIGS. 12 and 13, reference numeral 140 denotes a source follower resistor, numeral 201 an n-channel MOSFET corresponding to a channel portion of the IGBT, numeral 202 an npn transistor composed of the drift layer, the channel layer and the emitter layer, numeral 203 a pnp transistor of an MOSFET composed of the collector layer, the buffer layer, the drift layer and the channel layer, numeral 204 an npn transistor of a MOSFET composed of the drift layer, the base layer and the emitter layer, and numeral 205 the lateral MOSFET.
In the conventional structure, a leakage hole current flows into the source electrode 114 of the MOSFET through the base layer 132. When the hole current flows into the source electrode, a voltage generated across the source follower resistor 140 becomes higher than a desired voltage, thereby causing an erroneous operation of the circuit.
The present invention is made taking the above problem into consideration and provides a circuit incorporated IGBT which can prevent an erroneous operation of a circuit.
A circuit incorporated IGBT according to the present invention is provided with a semiconductor substrate having an IGBT area and a circuit area which are adjacent to each other. In a semiconductor layer of one conductivity type in which a circuit element is formed in the circuit area, there is provided another semiconductor layer of another conductivity type which adjoins the circuit element and has an impurity concentration higher than that of the semiconductor layer of the one conductivity type. An electrode contacts such other semiconductor layer and this electrode is connected to an electrode of an IGBT in the IGBT area.
According to the present invention, carriers are ejected from the second semiconductor layer to the electrode of the IGBT, thereby making it possible to prevent an erroneous operation of a circuit.
The one conductivity type may be p type or n type. The electrode of the IGBT is, for example, an emitter electrode. Carriers are holes or electrons.